(a) Field of the Invention
The present invention relates to an adaptive transmission method in a wireless communication system using frequency division duplexing. More specifically, the present invention relates to an adaptive transmitting and receiving device and method in a wireless communication system using frequency division duplexing for feeding back a small amount of information to effectively perform adaptive transmission in an environment using at least one transmit antenna and at least one receive antenna.
(b) Description of the Related Art
As shown in FIG. 1, an encoder and modulator 180 of a transmitter 100 transmits pilots or preambles 130 to a receiver 110 through a wireless channel 120, and the receiver 110 uses a demodulator and decoder 140 to demodulate them, measures a (per-carrier) SNR (signal to noise ratio) 160 through an SNR measuring device 150, and feeds this information back to the transmitter 100 in the wireless communication system using conventional frequency division duplexing. The transmitter 100 adaptively applies a modulation method, an encoding method, and power allocation through a modulation and encoding method determining unit 170 based on the SNR 160 fed back from the receiver 110 to allow the encoder and modulator 180 to adaptively transmit traffic data 190, thereby enhancing performance of the wireless communication system and increasing its capacity.
Differing from a single carrier system, it is difficult to apply the conventional method to the actual system due to the large amount of feedback information excluding the case of very slow changes in channels, because the channel gains of respective carriers are different in frequency selective fading channels in the case of a multi-carrier system. In particular, since a cellular system which uses frequency division duplexing supports fast mobile speed and has a lot of carriers, the amount of feedback information becomes very large, and hence, it is impossible to use the conventional method.
As shown in FIG. 2, therefore, a method for combining adjacent carriers into a predetermined number to use them as carrier groups 200, finding mean SNR 220 or lowest SNR 230 of the carrier groups from the actual SNR 210, and feeding them back is disclosed (refer to “Adaptive modulation techniques for duplex OFDM transmission,” by T. Keller and L. Hanzo, IEEE Trans. Vehicular Technology, vol. 49, no. 5, September, 2000; and “A blockwise loading algorithm for the adaptive modulation technique in OFDM systems,” by R. Grunheid, E. Bolinth, and H. Rohling, VTC 2001 Fall, IEEE VTS 54th, Vol. 2, pp. 948-951.)
Feedback information can be reduced by the number of carriers in the carrier groups in this case, but when the number of carriers in the carrier group is increased in the frequency selective fading channel, the channel gains are changed in a single carrier group to degrade the system performance, and when the number of carriers in the carrier group is decreased, the amount of feedback information is still large, and accordingly, it is not useful for the system that supports fast mobile speed.